A planisphere is a type of star chart
that can be set to show the location of objects in the sky for any given date
and time. It is small, portable and easy to use, providing a very convenient
tool for identifying objects seen in the sky, or locating specific objects of
interest. Planispheres are designed to be accurate for a specific observer latitude, but are useful over
a range of latitudes. The planisphere in Figure 1 is designed to be used for
latitudes from 30º to 40º North.

Figure 1

Basic
Features

Figure 1 shows the front side of a
planisphere. Bright stars, constellation outlines, and some deep sky objects
are shown on the white “sky” portion of the planisphere. This “sky” can be
rotated about the center of the planisphere, corresponding to the apparent
rotation of the night sky about the North
Celestial Pole. The star Polaris (the North Star) is located in the sky
very near the North Celestial Pole and can be found at the “center” of the
planisphere. The solid curved portion of the planisphere which covers a portion
of the bottom of the sky represents the horizon
(where the earth meets the sky). On the front of the planisphere, the center of the horizon corresponds to
the direction North (0º azimuth).
The right side of the horizon is the eastern
horizon (90º azimuth), and the left side is the western horizon (270º azimuth). To use the planisphere, it must be
oriented so that the planisphere horizon corresponding to the direction you are
facing is at the bottom. For example, the planisphere in Figure 1 is correctly
oriented for a person facing North.

Figure 2 indicates the location of
several additional reference points and lines on the planisphere.

Figure 2

The meridian is an imaginary line that runs from the North Celestial
Pole, through the zenith, to the South Celestial Pole. It corresponds to a line
connecting the centers of the two “grommets” or fasteners on the planisphere. The
zenith (the spot directly overhead
in the sky, altitude = +90º) lies on the meridian approximately in the center
of the “sky”. More precisely, it is located where the declination equals the
observer’s latitude.

Additional reference lines on the
planisphere include the celestial
equator, and the ecliptic. The
celestial equator is the solid circular line that near the edge of the
“sky”.The ecliptic is the curved dashed
line that intersects the celestial equator at two points (vernal and fall
equinoxes).

Motion
of the Sky

The “sky” portion of the planisphere
can be rotated either clockwise or counterclockwise.

Figure 3

As the night sky appears to rotate
around the north celestial pole due to the rotation of the earth, objects in
the sky that are not circumpolar rise above and set below the horizon
throughout the night. Circumpolar objects have declinations greater than (90º minus observer’s latitude) and never go
below the horizon and therefore never “set”. The direction of rotation of the
“sky” is easily determined by remembering that objects rise in the east and set
in the west (see Figure 3). To rotate the (facing north side) sky portion of
the planisphere to correspond to the rotation of the night sky, it must be
turned counterclockwise.

Using
the Planisphere

To use the planisphere, it must be set
to a specific date and time. The months and days throughout the year are marked
on the outer edge of the moveable portion of the planisphere (the “sky”) as
shown in Figure 4. Standard times are indicated on the outside edge of the
fixed portion of the planisphere below the horizon (add one hour to the printed
times for daylight savings time). To set the planisphere to display the sky for
a specific date and time, rotate the “sky” until the desired date and time are
aligned. In Figure 4, one date and time at which the sky will appear as shown on
the planisphere is April 20 at 9:00 PM
(local standard time; 10 PM
local daylight time).

Figure 4

There are many
combinations of times and dates shown on the planisphere in Figure 4 (For
example:March 5 at 12 midnight, and December 18 at 5 AM. As the days progress throughout
the year, the same configuration of the sky will appear at earlier times.

With the
planisphere set to a specific date and time of interest, orient it with the
azimuth direction (N, E, or W) that you are facing at the bottom when viewing
the sky. The “sky” on the planisphere will then correspond to the night sky you
are facing. To view the southern sky, the back side of the planisphere is used
(see Figure 5).

Figure 5

The back side of the planisphere
provides a view of the southern sky which has less distortion than the front
side.Note that when facing south, the
eastern horizon is on the left side, and the western horizon is on the right
side of the planisphere.

Determining
Rise, Transit, and Set Times

To determine when an object will rise on a specific date, rotate the planisphere
“sky” until the object is aligned with the eastern horizon. Then read the time
of day corresponding to the specified date. Figure 6 illustrates the star Vega
has just risen and is above the horizon at 9 PM on April 20. It also has just
risen at Midnight on March 5.

To determine when an object will set on a specific date, rotate the planisphere
“sky” until the object is aligned with the western horizon, and read the
corresponding time for the specified date. Figure 6 illustrates that the star
Rigel and the constellation Orion are setting at 9 PM on April 20.

An object is said to “transit” when it
crosses the meridian. To determine when an object transits, set the object on
the meridian (on the planisphere, the meridian is on a line connecting the two
grommets) and read the corresponding date and time. In Figure 6, the
constellation Leo is on the meridian, and the star Regulus has just crossed the
meridian at 9 PM on April
20.

Figure 6

Additional
Planisphere Features

Figure 7 illustrates several
additional features of the planisphere. For example, the celestial coordinate
system coordinates of Right Ascension (RA) and Declination (Dec) are marked on
the planisphere.

Right
Ascension values are
marked at one hour intervals by “tick marks” on the celestial equator. Radial
lines from the North Celestial Pole mark RA values at three (3) hour intervals
(0, 3, 6, 9, 12, 15, 18, and 21 hrs RA). Although the values of RA are not
labeled on small planispheres, the line corresponding to 0 hrs RA can be identified by locating the vernal (spring) equinox. The vernal equinox is located at the point
where the ecliptic crosses the celestial equator in March. The line
corresponding to a value of RA = 12 hrs
corresponds to the fall equinox. The
fall equinox is indicated on Figure 7. RA values increase in a clockwise
direction on the celestial equator.

Figure 7

Declination
values are marked at
10º intervals along the radial lines of RA. Values of declination can be
determined by remembering that the declination at the North Celestial Pole is =
+ 90º. Declination values decrease as you move away from the North Celestial
Pole and toward the Celestial Equator.

As an example, using the planisphere
in Figure 7, the celestial coordinates ofthe star Arcturus can be estimated to be approximately RA = 14 hrs 20
minutes and Dec = 20º.

Apparent (visible) magnitude, or
brightness of stars shown on the planisphere correspond the size of the dot
indicating the star. The larger the dot, the brighter the star will be in the
night sky.

Finally, notice the gray “smudging”
that appears in each of the figures. This “band across the sky” represents the
Milky Way.